Re: Contactor question, Arc Suppression

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>

Hi,

At 04:17 AM 8/16/2005, you wrote:
> OK I'll bite. I can see how this might work with in
> theory, But how to you evergize a contactor coil at
> zero crossing when the voltage you're using to control
> the coil is also at zero???

It would probably need it's own DC power supply.

> Also, since you cross zero
> 120 times per second, or every 0.0083 seconds, and
> each event really only lasts a tiny fraction of that
> number, can you reliably close a contactor with that
> great a precision?

You could over volt the contact too!!  Hit the closing coil with say 
3X the voltage for an instant to really get it too close 
fast!!   Been there, done that :o)

> I believe you can tell it to start
> closing, with an adjustable "time to close" setting,
> but does the contactor really close in the same amount
> of time, every time? We're talking fractions of a
> millisecond tolerance here.

It might need a little micro controller...

Like I said, I don't think it is worth all the trouble.  Easier just 
to get a big contactor rather than make a relay computerized controller :o))

Also, the contacts still have to be substantially over rated since 
current might not share well between the contacts.

It sounds like the RC snubber idea is a much better alternative.

I suppose on could immerse it in oil like the big boys do...  But 
that is just another messy complication...

Really, you need the "right" sized contactor in the first 
place...  Maybe E-bay...

Cheers,

        Terry


> Adam
>
> --- Tesla list <tesla@xxxxxxxxxx> wrote:
>
> > Original poster: Terry Fritz
> > <vardin@xxxxxxxxxxxxxxxxxxxxxxx>
> >
> > Hi,
> >
> > There is a "trick".  I am not sure it is worth all
> > the trouble...
> >
> > You make a circuit the monitors the current and
> > voltage along with
> > some delay electronics.  The idea is to only open
> > the contactor when
> > the current passes through zero.  Close it as the
> > voltage passes
> > through zero...
> >
> > Then, the contacts only have to take the nominal
> > current rather than
> > peak or break situations...  But the circuit and all
> > the trouble it
> > takes to make it has to be balanced against just
> > getting a really big
> > contactor in the first place...
> >
> > It can be done with some simple stuff in very
> > controlled situations,
> > but it is sort of an "art form"...
> >
> > An alternate is to get contactors with fail safe
> > switches that detect
> > a failed contact.  If it welds closed, a warning
> > goes out...
> >
> > Contactors weld closed all the time...  Probably
> > their #1 failure mode....
> >
> > Maybe best to use really big hydraulic circuit
> > breakers...  They know
> > how to hand the big currents ;-))
> >
> > Cheers,
> >
> >       Terry
> >
> >
> >
>
>
>
>
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